As I was helping my son revise for his coming exams, there was some confusion over verb agreement when it came to the use of either and neither. Fortunately, he had a book that helped explain the rules. So now I won't stumble over this one anymore. Here are the rules.
Rule 1: We use singular verbs with either of and neither of. (Note that we're pairing either and neither with of.) The reason is simple. We're choosing between two single subjects so therefore, the verb needs to be in singular form.
Either of the girls has eaten the cake.
(I.e., one of the girls has eaten the cake.)
Neither of the girls has eaten it.
(I.e., none of the girls has eaten the cake.)
Because there are only two girls in question, the first one or the second one ate the cake. So the verb is in singular form.
Rule 2: When we use either... or or neither... nor to join subjects of different numbers, we apply the rule of proximity. This rules states that the verb agrees with the subject that is nearest to it.
Either Tim or Jane knows the answer.
Neither Tim nor Jane knows the answer.
Either the girl or the boys have to finish the task.
In this example, the boys are closer to the verb so it takes on its plural form: have. But if we switch the subjects around...
Either the boys or the girl has to finish the task.
Now the girl is closer to the verb so it takes on its singular form.
Either the teacher or the students know the answer.
Either the students or the teacher knows the answer.
Neither the apple nor the bananas are ripe.
Neither the bananas nor the apple is ripe.
That has cleared it up once and for all for me. The thing I watch out for when I write is the word of. If I'm using either of or neither of, then I'm talking about a choice between two single subjects and so I use the singular form of the verb. If I'm using either... or or neither... nor, then I'll use the rule of proximity to help me decide on the verb form. Easy!